This invention relates to a monolithic carrier catalyst and an arrangement of such catalysts, wherein the catalysts are well known in the art. The catalyst of this invention is useful in the catalytic purification of exhaust gases from internal combustion engines, and makes it possible to avoid premature catalyst aging resulting from damage of the catalytically active phases, especially where such damage is caused by catalyst poisons.
The exhaust gases emitting from a continuously increasing number of motor vehicles has caused a very real air pollution problem. Of utmost importance in the solution of this problem are catalytic processes wherein noxious substances are converted by catalytic action into substances that can be tolerated in the atmosphere. The catalysts employed in such processes must meet stringent requirements, such as catalytic effectiveness, long duration of catalytic action and mechanical stability. In addition to pellet catalysts, above all monolithic carrier catalysts have been employed. These monolithic carrier catalysts can be produced from cordierite, mullite, aluminum oxide, silicon carbide or metal alloys. These monolithic carrier catalysts are characterized by a honeycomb structure and comprised of channels running parallel to the direction of flow of the exhaust gases. The monoliths cause no significant loss of pressure; however, the cross-section of the channel and the shape of the channel can influence the flow of the exhaust gas and thus the catalytic effectiveness of the system.
The catalytically active substances, such as noble metals, oxidic compounds or combinations of these substances, are well known in the art, and are generally applied in thin layers on the monolithic carrier. Low surface area skeleton bodies can be provided with a very thin, high surface area intermediate carrier layer in order to assure better distribution of the catalytically active phase on the carrier body. The oxides of copper, chromium, manganese, iron, cobalt, nickel and mixtures of these substances, such as copper chromite, are well known for use as non-noble metal catalytically active compounds. Among the catalytically active noble metals that can be employed are platinum, palladium, rhodium and ruthenium. It is also known that non-noble metals can be doped with the noble metals, or vice versa. Similarly, compounds of non-noble metals and noble metals can also be employed. In many cases, relatively small quantities of other elements can be added to the noble metals and non-noble metals mentioned above. These other elements are termed promoters for the improvement of certain characteristics of the catalyst system. Among these other elements are alkaline earth metals, such as magnesium, calcium, strontium or barium; rare earth, such as samarium, lanthanum and cerium; and elements from the fourth group of the periodic system, such as titanium and tin. It is known that the catalytically active phases can change during operation because of aging of the catalyst or poisoning of the catalyst.
The effectiveness of the catalyst can be reduced by relatively high operating temperatures, but above all by the fuel and motor oil additives or their decomposition products. Typical of such harmful substances are lead alkyl compounds (anti-knock agents), halogen alkyls, phosphorus, sulfur and zinc compounds. Genuine catalyst poisoning can result when lead compounds, which are contained in the exhaust gases in the form of aerosols, damage noble metal-containing catalysts.
There exists a need in the art for a monolithic carrier catalyst or arrangement of monolithic carrier catalysts for the purification of exhaust gases from internal combustion engines, whereby the catalyst is not subjected to premature aging and catalyst poisoning.